A standard variable-pitch propeller has a rotary hub from which a plurality of blades project radially. These blades are each pivotal about a respective normally radial axis to change their angle. Different angles are more efficient at different speeds so that normally as speed increases the angle is changed appropriately. In addition reversing the pitch allows the boat provided with the variable-pitch drive to move backward.
In order to change the blade angles the relative axial positions of a coaxial core and sleeve of the propeller or of a propeller hub and respective blade rods are varied. This can be done manually in systems where the setting is only changed occasionally, but for boats where the angle is changed while the propeller is in use this is typically done by a hydraulic actuator. U.S. Pat. No. 4,880,402 describes such a system where the propeller-pitch actuator is provided inside or on the boat hull immediately upstream of the propeller. In this arrangement the outer sleeve is shifted to change pitch.
Commonly owned and copending U.S. patent application Ser. No. 07/635,739 (now U.S. Pat. No. 5,073,134) describes an arrangement where the cylinder is mounted immediately upstream of the propeller in its mounting bearing. Here the piston rod of the hydraulic cylinder projects axially through the variable-pitch propeller and is connected on the downstream side of the propeller with the adjustment rods.
The disadvantage of these systems is that they must all be incorporated in the variable-pitch propeller right from the start. They cannot be retrofitted to a variable-pitch propeller and each such actuator can only be used with a specific propeller. Servicing and/or replacing such an adjuster requires dismantling of the entire propeller assembly.